Variable band width circuit



Nov. 27, 1951 P. R. BELL, JR

VARIABLE BANDWIDTH CIRCUIT Filed May 1946 OUTPUT FROM STAGE FREQUENCYINVENTOR PERSA R. BELL JR.

BY W44 ATTORNEY Patented Nov. 27, 1951 2,576,329 I I new) w n rnfcm'ourrPersa E. Bell, Jn, Oak Ridge, Tenm, assignor, by mesne assignments, tothe .United; States of Americaas represented by theSecretary of theThis. invention relatesto electronic amplifier circuits, and moreparticularly to amplifiers having variable pass band characteristics-.

" Itis well known intheart that an electronic amplifier will provideessentially equal amplifica- 'tion' -of all signals having-a frequencybetween two limitsgwhich frequency limits specify the pass bandgorbandwidth, of the amplifier. For certain applications it maybe desirableto have a narrow-"bandwidth amplifier to reduce" the amount ot Iextraneous signal; known as noise, and permit the successfulamplification of lower level input-signals.=- In other applications,faithful reproduction of the input signal may require an-increasedbandwidth. Since, in general, the

bandwidth of an amplifier cannotbe-increased without a correspondingdecrease-in-- the gain, the design of an amplifier with-variablebandwidth becomes quite diflicult. In the present invention an amplifiercircuit is provided 'in which the bandwidth may be varied, with the gainbeing held essentially constant.

"Accordingly, it is a primary purpose-of this invention togenerally'improv'e electronic amplifier'circuits.

It is also an object of thisinvention to provide an electronic amplifierin which the band of frequencies which may be successfully amplified isvariable. a I a It is further an object of this invention to provide anelectronic amplifier circuit in which the bandwidth may be varied"without essential change in'the'gain' of the" amplifier tagei .i :'Iheif oregoingand othenobjects of theinvention will become apparent"upona careful. consideration of the detailed description when takenwith the accompanying drawings in which:

Fig. 1 is a circuit diagram showing one embodiment of the invention; and

Fig. 2 shows representative frequency response curves obtainable by thisinvention.

Referring to the drawings and more particularly to Fig. 1, the inventionis shown in relation to other components of an amplifier stage. Theinput signal at terminal H is applied to the control grid of electrontube l2 which is shunt connected to a source of positive voltage atterminal l3. The output of this stage is coupled to a second amplifierstage consisting of electron tube l4, and the output is taken fromterminal I5. An analysis of the operation of the coupling circuit is asfollows. Variable inductance H5, inductance H, and variable resistancel8, together with stray capacitance [9, shown by dotted lines in Fig. 1,that shunts these elements to ground,

'- Application Ma 3 ,-1946,'SerialNo.666,876

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forms a parallel .resonant circuit. uThe Q, or quality factor, of thisparallel resonantmlrcuit can be adjusted by means of, resistance.

When'the valueof'resistance 18. is small, the Q of the parallel resonantcircuit. is largeandthe pass band of-the circuit is narrow. .Whenresistance -l8- ismade larger,- loading the resonant ci-rcuitptheQdecreases, and the pass. bandv increases in width. Assuming thatinductance H is equal to-zero, then as the value-of resistance I8 isreduced from a large-value #to a low value, the voltagecoupled tothe'secondary 0011129, theoretically remains constant; However; inactual practice, at ve'rylow-values ofresistance l8, the voltage coupledto the secondary coil 20 falls'ofidue to tube and coillo'ssea- Byincluding the inductance l1, this dropin the voltage coupled. to co'il'ill can be eliminated because as resistance [8 is decreased, the Q ofthe parallel resonant circuit increases, which-inturn increases the.voltage which is coupled" to secondary coil 20..It is apparent thenthat the voltage which iscoupled to thefseco'ndary coil M depends notonly son the valueofresistance I8 but also on'the valueiof inductancell. By proper selection of the value of inductance 'I'Lthegain of thecircuit-for low values of resistance l8, that'is; the high-Q condition,can bemade' any desired value relative to the gain'in thelow Qcondition. After proper selection'of inductance ll; then, resistance l8serves as a control of the bandwidth of the amplifier withoutserioussacrifice in gain of thecircuit.

Part 2| of secondary coil 20 forms an impedance coupling linkbetween'inductance'lli and inductance 20. the coupling of these twocoilsis ma'de g re'ater by increasing the'relative 'size of part 2| ofinductance 20,:the pass band curve will tend to become double peaked.The extent of these double peaks can be controlled by varying the valueof part 2| of inductance 20.

The center frequency of the band-pass curve for the circuit can beadjusted by means of variable inductance IS in the normal fashion.

Fig. 2 shows representative frequency response curves of the output ofthis circuit. Curve 22 is the frequency response curve for the circuitwhen the value of resistance 18 is low, that is, in the high Qcondition. Curve 23 illustrates how the pass band is broadened when thevalue of resistance is increased. It is shown by these curves that thepass band can be appreciably broadened without serious sacrifice ofgain.

The invention described in the foregoing Specification need not belimited to the details shown.

- 3 which is considered illustrative of one form the invention may take.

What is claimed is:

1. A variable bandwidth coupling circuit for electronic amplifierscomprising, a variable inductance, a fixed inductance, a variableresistances-maid; elements connected series in the order named, thevself-capacitance- 'oi said elements in combination with said inductancesforming a parallel resonant circuit, and a second:

fixed inductive element impedance coupled to said resonant circuit atthe junction of said first" fixed inductance and saidvariablerinductancer the magnitude of said first fixed: inductancedetermining the relative gain of said coupling circuit at differentbandwidths, the magnitude of said variable resistance substantiallydeterminput circuit, a network coupling the output circuit of the firsttube to the input circuit of the second tube comprising, a variableinductance, a first fixed inductance and a variable resistor seriallyconnected in the order named in shunt with said first tube, theself-capacitance of said elements in combination with said inductancesforming a parallel resonant circuit, the Q of said resonant circuitbeing adapted to be raised and I lowered by decreasing and increasingrespectively the value of said resistance, a second fixed inductancecoupled to the input circuit of the second tube,,andga conductorconnecting the junctiOnptsaid variable inductance and said firstfixed-inductance to said second fixed inductance, the inductance of saidfirst fixed inductance being of such. a magnitude that the voltage ingthe pass band of said coupling circuit, and

the magnitude of said variable inductance substantially determiningthecenter frequency of saidipass band. ofv said coupling circuit.

2..In a radio frequency amplifiena variable bandwidth coupling circuitcomprising, a variable inductance, a first fixed inductance andavariable.- resistor' serially connected in the order named, theself-capacitance of said inductances and resistor incombination withsaid inductances and resistor forming a parallehresonant circuit, asecond fixed inductance, and a conductor connecting, the. junction ofsaid variable inductance and said first fixedinductance to saidsecondifixed inductance, the magnitude of said firstzfifixed inductancedetermining the relative gain of said coupling circuit at difierentbandwidths, means for varyingsaid resistance to control the passband ofsaid.circuit,.and means foradjusting the value of said inductance forcontrolling the center frequency of the pass bandof-saidzcouplingcircuit, v

3. In aradio frequency amplifier, at least two electron tubes eachhaving input and outputcircuits,a ne twork'coupling the output circuit.of the first tube to the input circuit of the second tube comprising, avariable inductance, a-first fixed inductance and a variable resistorserially connected in the order named in shunt with said I first tube,the self -capacitance of said" elements in combination with saidinductances forming, a parallel resonant circui't, a second fixedinductance coupled to the inputcircuit of the second tube, and aconductor coupling the junction of .said first fixed inductance andsaidvariable inductance to a desired point on said second fixed inductance.

.4. In a radio frequency amplifier, atle'ast'two electrontubes eachhavingv an input and? ano'ut' coupl'edto said second fixed inductance issubstantially constant with variations in magnitude of said variableresistor.

. 5; Ina radio frequency amplifier, at least two electron tubes eachhaving an input and an output circuit, a network coupling the outputcircuit of: the first-tube to, the input, circuit of the secondtubeicomprising, a variable inductance, afixed inductance and a variableresistor serially connected in the order named in shunt with said firsttube, the stray capacitance of said element in shunt therewith formingaparallel resonant circuit, the bandwidth of said coupling circuit beingadjustable byvariations in the magnitude of said resistor, a secondfixed inductance coupled to the inputcircuit of the-second tube, and aconductor connected between the junction of said first fixed inductanceand said variable inductance andan-adjustable tap on said;second fixedinductance, the magnitude of said first variable inductance determiningthe center frequency of the pass band of said network and the inductanceof said first fixed. inductance being of such a magnitude that thevoltage coupled to said second tube are substantially constant withchanges in bandwidth caused by variations in the magnitude ofsaidresistor. I I

PERSA R; BELL, Jr.

Number Name Date 2,051,898 Roberts." Aug. 25; 1936 2,052,703 FarnliainSept. 1, 1936 2;4042270 Bradley 'July 16; 1946

